Does Vi Peacekeepers still stun if she gets bounced and no longer at that battlefield?

No — if Vi is bounced before her attack trigger resolves, the stun fizzles.

Her ability reads "When I attack, [Stun] an enemy unit here." The word "here" requires Vi herself to still be at that battlefield when the ability resolves. If she's returned to hand (e.g., Star-Crossed or Flash) in response to the trigger, the chain resolves last-in-first-out — the bounce resolves first, and when her trigger tries to resolve it can't reference her location, so no unit gets stunned (FAQ #10045, #10627). Same applies if she's killed instead of bounced.